The present invention relates generally to current controlled oscillation (ICO) devices, and, more particularly, to a design structure for an improved ICO device having wide frequency range and integrated proportional frequency control capable of operation at low supply voltages.
Controlled oscillators are used in a variety of integrated circuits for applications such as, for example, signal generation and detection, as well as in phase locked loop (PLL) circuits. In a controlled oscillator, the frequency of an output signal is responsive to a control signal provided thereto. There are various types of controlled oscillators, with one of the more common types being a current controlled oscillator (ICO). A typical ICO includes a controlled current source coupled to a ring oscillator, which in turn features a chain of inverter stages (typically an odd number). The output of one inverter in the stage is coupled to the input of a succeeding inverter, and so on, with the output of the last inverter fed back to the input of the first inverter in the stage. Typical inverters are formed from CMOS transistors, although other types of devices may also be used.
The frequency of the output signal of an ICO is inversely proportional to the delay/switching time of the inverters that form the oscillator. In turn, the switching time of an inverter corresponds to the time needed to charge and discharge the input capacitance of a successive inverter to a level respectively above or below the switching threshold of the successive inverter. The charge and discharge period is determined by the magnitude of current that is used to charge the input capacitance. It is this charging current that is provided and controlled by the controlled current source.
One disadvantage associated with conventional current controlled oscillators is that the gain of the oscillator is not controllable at a given operating frequency. This is particularly the case for an oscillator that is used over a wide range of operating frequencies, as the gain of a conventional ICO (i.e., the relationship of output frequency versus bias current) is constrained by the desired frequency range. It would therefore be desirable to be able to configure a wide range oscillator for applications such as phase locked loops, for example, that also keeps the gain at a low level so that the overall loop bandwidth can be optimized.